The invention relates to a power tool, in particular to an angle grinder, comprising a quick clamping device for fastening of a tool onto a spindle, comprising a clamping flange and a counter flange, between which the tool under the effect of an elastic force means is clamped, an actuation element being movable between a clamping position and a release position, wherein in the release position the clamping between the counter flange and the clamping flange against the effect of the elastic force means is neutralized, and a cam acting on the clamping flange which can, at least in the release position, be rotated by a motor of the power tool, and on which acts at least, when the actuation element is transferred from the clamping position in the release position, a running surface designed on the actuation element, wherein a movement of the actuation element is transferred into an axial shift of the clamping flange; and which, in the release position, when the motor is idle, holds the actuation element under the action of a frictional force acting between the cam and the running surface.
Such a power tool is known from EP 0 152 564 A2. This known power tool has a quick clamping device, by means of which disk-shaped tools, e.g. abrasive wheels or circular saw blades, can be exchanged quickly and comfortably. The quick clamping device comprises, for that purpose, a clamping flange constructed as a nut, and a counter flange, between which the disk-shaped tool is clamped. In this way, the clamping flange is screwed upon a clamping pin, which is attached for common rotation to a hollow spindle driven by the motor of the power tool, but movable in axial direction. The clamping pin is clamped, with respect to the hollow spindle, by means of a spring in such a way that the clamping pin is pulled toward the counter flange, due to the bracing effect of the clamping flange.
By transferring an actuation element, which is configured as a pivot lever, from a clamping position into a release position, the clamping between the clamping flange and the counter flange can be neutralized. To this end, a cylindrical socket is arranged on the pivot lever, said socket being screwed into the housing of the power tool. If the pivot lever is actuated, the socket is screwed further into the housing, until it, finally, acts upon the end of the clamping pin facing towards the socket and presses down the clamping pin, together with the clamping flange screwed thereon. In that way, the clamping between the clamping flange and the counter flange is neutralized, so that the clamping flange can be unscrewed from the clamping pin manually. After that, the tool can be exchanged against another tool.
In the operation of such power tools it has turned out that users sometimes switch on, carelessly or curiously, the motor of the power tool, although the pivot lever is still in the release position. It is true that also when the clamping flange is only loosely screwed upon the clamping pin, this cannot result in detaching the tool and, thus, in endangering the user. As, however, the dog is still in its declined position, it presses, with its bottom part, from the top onto the clamping pin, which now, after switching on the motor, rotates with high speed. Due to the relatively high forces, welding or deformation may occur in this case.
In the quick clamping device known from EP 0 152 564 A2, the friction conditions between the areas facing each other of the dog are selected in such a way that, when the motor is started, the friction force between the two areas is high enough to transfer the pivot lever into the clamping position. The pivot lever is, thus, if such an operating error occurs, returned into the clamping position.
It has turned out, however, that this return movement is relatively hard to monitor. On the one hand namely, by starting the motor, a relatively high torque is transmitted onto the pivot lever, as the adhesive friction between the two engaged parts, existing at the beginning, allows a high force transmission. Thus, the self-instructed return process starts with a very abrupt movement, which may result in accidents.
On the other hand, the friction conditions between the two engaged surfaces change after a while, as the force transmission is performed exactly by making use of the friction force and, thus, a wear of the surfaces is inevitable. The result is that also the force transmission and, in connection therewith, the kind of movement of the pivot lever changes after a while.
From EP 0 650 805 B1, another power tool is known, which is equipped with a similar quick clamping device. The actuation element is in this case, however, configured as a pivot lever, which is firmly connected with an eccentric. When the pivot lever is pivoted, the eccentric presses down a pressure head which is guided axially movable, until the eccentric rests on a thrust piece, into which the clamping flange is screwed via a threaded bolt. If the clamping lever is further pivoted, the pressure head finally presses down the thrust piece and, thus, also the clamping flange, against the action of cup springs.
In this known power tool, the pivot lever is connected via a shifter bar with a switch for starting the motor in such a way that the motor can be switched on only when the actuation element is in the clamping position. With this measure, it is prevented that, if the actuation element is in the release position, the lowered pressure head presses, with its bottom part, onto the thrust piece, which would rotate at high speed after starting the motor. Without such a measure, there would be welding or deformation between the pressure head and the thrust piece (or a friction plate fastened onto it), due to the relatively high forces. The mechanical connection between the actuation element and the switch for switching on the motor is, however, relatively complicated in design.
It is thus an object of the invention to disclose an improved power tool which overcomes the drawbacks of the prior art. It is a further object of the invention to provide a power tool that reliably avoids failures when the motor of the power tool is switched on erroneously. The power tool shall be nevertheless simple in design, while allowing a cost effective manufacturing.
With respect to such a power tool as mentioned at the outset, this object is achieved by mating the cam and the running surface in such a way that, in the release position, a rotation of the cam caused by starting the motor reduces the frictional force between the cam and the running surface in such a way that the cam, which was prestressed by elastic force means, moves the actuation element from the release position into the direction of the clamping position on its own.
The solution principle underlying the invention is, consequently, distinguished by the fact that, for restoring the actuation element in its clamping position, not a torque generated by the motor is used, but, rather, the pressure exerted by the elastic force means via the cam onto the actuation element is used. As long as the cam rests, however, the adhesive friction force acting between the cam and the running surface keeps the actuation element in the release position. Only if the cam, when the motor is started, is set into rotation, the adhesive friction transitions into a distinctively smaller sliding friction, which is, then, not sufficient any more to keep the actuation element in the release position. The motor causes, thus, merely a modification of the friction conditions between the cam and the running surface; a force transmission from the motor to the actuation element, however,xe2x80x94at least in a significant amountxe2x80x94does not take place.
In comparison to the clamping device of EP 0 152 564 A2 mentioned at the outset, this principle has the decisive advantage that the speed of the actuation element, when being returned, is practically not dependent any more on the motor speed, but only depends on constructively determinable, mostly unchangeable parameters.
These parameters are, in particular, the pressure force caused by the elastic force means, the moment of inertia of the actuation element, the direction in which the cam acts on the actuation element via the running surface, and, of course, the friction conditions between the cam and the running surface. The latter depend, on their part, on the type of the tools used as well as on their surface quality.
In this regard, by the way, a clamping flange and a counter flange shall be regarded as any component suitable to clamp a tool onto a shaft. The clamping flange can be, in particular, a common nut, which is screwed onto the shaft.
In this regard, the terms clamping position and release position are not to be understood in a limiting sense, designating an exactly defined position. The clamping position shall rather be any position, i.e. also a greater position area of the actuation element, in which at least a partial clamping of the clamping flange and the counter flange is obtained. Correspondingly, all positions are designated as release position, in which there is no clamping between the clamping flange and the counter flange. So, if a movement of the actuation element from the release position in direction to the clamping position is the subject, this means, finally, that the actuation element is moved so far until clamping takes place at least partially. This results, on the other hand, in a relief of the components of the quick clamping device, so that additional forces generated by the motor can practically not result in damage any more.
A cam shall be any component, which engages, for the purpose of force transmission, a surface provided on another component, here called running surface. A special shape is not to be implied with the term cam. The running surface itself can be plain, but also be curved optionally, wherein the curves can also be considered as inclinations of a running surface unwound.
In this regard, it is particularly preferred, if the running surface has at least two sections of different inclinations.
This has the advantage that, by variation of the inclination of the running surface, the movement behavior of the actuation element, when returning on its own into the clamping position, can be influenced within certain limits by construction. Namely, the inclination influences the direction in which the cam acts upon the actuation element via the running surface. For instance, a setting of the running surface inclination is possible, in which the actuation element is transferred from the release position into the clamping position at approximately constant speed.
In a preferred improvement of this embodiment, the inclination of the running surface is smaller in the release position of the actuation element than the inclination of the running surface in the clamping position.
Thus, a small force transmission onto the actuation element at the beginning can be obtained, so that the return movement into the clamping position can be initiated slowly. Thereafter, the inclination of the running surface increases, so that the actuation element more accelerates. This can e.g. be advantageous, if the actuation element can be locked in the clamping position. The speed of the actuation element may then be sufficient to overcome the detent resistance.
In another advantageous embodiment of the invention, the running surface has at least two sections of different surface qualities.
This has the advantage that, in that way, also after the return movement has started, the friction conditions can be influenced. E.g. the running surface can be provided with a roughened section, which increases the sliding friction in such a way that a self-acting return pivoting of the actuation element is retarded, or at least a further acceleration is counterbalanced. The surface quality can also be modified by coating.
In a particularly preferred embodiment of the invention, the running surface is the circumferential unroll area of an eccentric arranged on the actuation element, mounted pivotably about a pivot axis.
This has the advantage that the running surface is so-to-speak rolled around the eccentric and, thus, has an essentially smaller xe2x80x9cspace requirementxe2x80x9d than a plain surface, as it may be e.g. constructed on an actuation element constructed as a slider. For example, the actuation element can have a pivot knob arranged laterally on the eccentric, by which the eccentric can be pivoted around its axis.
In a preferred improvement of this embodiment, the actuation element, however, comprises a pivot lever, which is fastened on the eccentric and is pivotable about the pivot axis of the eccentric.
The use of a pivot lever has the advantage that much higher torques can be applied than possible with a rotation knob, for instance. Moreover, by the sweeping pivot movement of the pivot lever in the self-acting return movement from the release position into the clamping position, it is clearly shown to the user that he has omitted to bring the pivot lever, before actuating the power tool, into the clamping position. The fixation of the pivot lever on the eccentric can also be performed via a free running. When the actuation element is in the release position, only the eccentric moves back to the clamping position, but not the pivot lever, when the motor is started.
In another advantageous improvement of this embodiment, the eccentricity of the eccentric is between 1% and 20% of the largest diameter of the eccentric.
It has been found that with such a selected eccentricity of the eccentric a particularly reliable force transmission according to the principle of the invention is possible.
In another preferred embodiment of the invention, the eccentric is arranged laterally displaced with respect to the pivot axis of the cam, into the direction in which the pivot axis of the eccentric extends.
Such a transverse displacement results in that, when starting the motor in the release position, the rotating cam transmits an additional torque onto the eccentric, which is not due to the elastic force means, but to the rotation of the cam as such. Whether this additional torque supports or counteracts the torque produced by the elastic force means, depends on the side to which the eccentric is arranged, displaced, in the direction of the eccentric pivot axis. Preferably, the displacement is selected so that the torque produced by the elastic force means is supported for reaching an additional acceleration of the actuation element.
In a further preferred embodiment of the invention, the surfaces of the cam and of the running surface have a Vickers hardness of more than 54, preferably about 64, and a surface roughness Rz of 0.2 xcexcm to 8 xcexcm.
In that manner, due to the great hardness, a sufficient wear resistance is provided, so that the friction conditions between the cam and the running surface, which are decisive for returning the actuation element, remain constant in the course of time. On the other hand, a roughness selected in such a way provides for a sufficient adhesive friction force between the cam and the running surface, so that the actuation element, in the release position, when the motor is idle, can be held by the cam. For example, such a surface quality can be reached by hardening and grinding, or by tumbling steel parts (if necessary, sintered).
In another advantageous embodiment of the invention, the surfaces of the cam and of the running surface consist of a porous sinter material, the pores of which near to the surface are filled with a lubricant.
By this measure known per se, an emergency running lubrication between the cam and the running surface is guaranteed. When the surfaces are worn, namely, the pores open gradually, thus releasing the lubricant contained therein.
In another preferred embodiment of the invention, the spindle being driven by the motor is constructed as a hollow spindle. The cam acts on the clamping flange via a thrust piece, to which the clamping flange is fastened detachably, the thrust piece being arranged movably in axial direction in the hollow spindle by the cam engaging the thrust piece against the effect of the elastic force means.
By this measure known per se, a very reliable quick clamping device can be realized in a simple way. The cam can be part of the thrust piece or can be firmly connected thereto, so that each movement of the actuation element is transmitted directly onto the thrust piece and, thus, onto the clamping flange fastened thereon. In this case, however, a small distance should be kept, at least in the clamping position, between the running surface and the cam, so that, during operation of the power tool, there will not be permanent friction between the rotating cam and the running surface of the actuation element.
In an advantageous improvement of this embodiment, the cam is, however, not directly connected to the thrust piece. The cam is, rather, part of a pressure head, which is mounted rotatably in a sleeve and, when the actuation element is transferred into the release position against the effect of said elastic pressing means, which presses the cam of the pressure head against the running surface of the actuation element, the pressure head is moved in such a way in the direction of the thrust piece that the pressure head comes at least indirectly in friction contact with the thrust piece.
This has the advantage that, on the one hand, the cam is always in touch with the running surface of the actuation element. This gives a better feeling when operating the actuation element than if it would suddenly be placed with its running surface on the cam. The friction contact between the pressure head and the thrust piece facilitates in particular the exchange of the tool, since also the hollow spindle being connected to the thrust piece for common rotation is blocked against rotation due to the friction contact. In order to enlarge the friction between the pressure head and the thrust piece, the latter can be, on its end, connected to a friction plate. This friction plate has a surface with a sufficiently high coefficient of friction and can be, should it once be worn, exchanged relatively easily against a substitute plate.
In another advantageous embodiment of the invention, an elastic return means acts onto the actuation element, which, independently of a rotation of the cam exerts a force onto the actuation element, which supports a movement of the actuation element from the release position into the direction of the clamping position.
By means of such an additional return means, it is reached that the actuation element also occupies a clamping position definable by a stop, when the actuation element is in an interposition between clamping position and release position. This embodiment makes also sense, in particular, in such embodiments, in which no pressure head admitted by elastic force means acts upon the actuation element, as was described above.
It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention.